A known type of digital x-ray imaging system, directed particularly to vascular-imaging, includes an x-ray source for directing x-rays through a patient to be examined, and an image intensifier tube aligned to receive a pattern of x-rays emergent from the patient's body. The image tube converts received x-rays to a corresponding visible light image. A television system views the light image and produces a set of analog signals describing that image. A digital angiography subtraction unit (DAS) receives the analog signals, and digitizes them, storing the signals in memory. The memory address of a particular stored signal denotes the portion, or "pixel" of the image which is represented by that signal. The stored digital signals each bear information defining the brightness of the image at the corresponding pixel.
The digital angiography subtraction unit includes circuitry which, when programmed by appropriate known software, causes the digital angiography subtraction unit to process and enhance the digital signals in various known ways.
The digital angiography subtraction unit also includes digital to analog conversion means for reconverting the stored and/or enhanced digital signals to analog form, for display on an appropriate analog monitor.
An imaging system such as described above is embodied, for example, in a vascular imaging system designated the "DIGICON 260", manufactured and sold by Picker International, of Cleveland, Ohio, U.S.A. Other such systems include a DIGICON 160, and an ANGICON, also made and sold by Picker International. A known form of digital angiography subtraction unit is a product designated as "DAS 211", also sold by the above referenced Picker International.
In operating such a digital vascular imaging system, before a series of diagnostic quality images can be acquired with the system, the user should acquire one or more test images as well. These test images are sometimes called "technique" images, and are often said to be acquired by means of a "technique shot".
The technique images serve two purposes. They show whether the anatomy of interest is receiving a proper radiation exposure. Also, they show whether images to be taken later will have any "hot spots".
"Hot spots" are those portions of the image that are so bright that the digital imaging system becomes saturated, and cannot handle them properly, because signals depicting those spots are greater than the dynamic range inherently defined as a limitation of the system.
"Hot spots" occur mainly at areas of the anatomy which are air filled, such as the lungs, throat, etc. When radiation intensity is raised to a level high enough to penetrate, and therefore image, more dense portions of the body, such as the mediastinum, the resulting radiation is too intense in areas corresponding to air filled organs. Therefore, when the radiation is sufficiently high to image thick and dense portions of the anatomy, the radiation is too high to image the air filled regions properly. As pointed out above, the digital processing equipment has the inherent limitation that it cannot properly handle brightness information above a "saturation" level.
If there are any saturated areas, some type of x-ray absorber element is often placed between the source and the patient in the area of the "hot spot" so that saturation is eliminated. After the absorber is put in place, another technique image must be done to see if saturation has been eliminated.
Portions of absorber material having different shapes, thicknesses and compositions are kept available in "kits", to tailor the absorption characteristics to optimize the images.
In the past, only very limited image processing has been performed on technique images. The unprocessed technique image was usually displayed to an operator, who had to judge whether saturation existed in any portion of the image. The operator generally tried to judge saturation by visual inspection of the image, i.e., by looking for loss of detail that usually accompanies such saturation.
An aid to the operator has also been employed. In a known system, a set of six histogram plots corresponding to the brightness along six parallel spaced lines at different locations in the image is produced. An operator can couple information obtained from the histograms with visual inspection to ascertain whether saturation is present.
A problem with this approach is that many operators have difficulty interpreting exactly what the histograms tell them. Also, the histograms only show information for the six image lines to which they correspond. Obviously, there are sizable gaps in he image with no corresponding histogram information. In these gaps, the operator must look for a very subtle loss of image detail to judge if there is saturation. Even experienced operators sometimes have trouble making this judgment.
Additionally, the operators have difficulty relating hot spot location to the anatomy. They therefore have trouble deciding precisely where to apply the x-ray absorbers.
The net effects of these problems are that operators of such vascular imaging systems spend considerable time performing technique shots, and, even with multiple technique acquisition, many actual digital study runs still have information loss due to areas of saturation.
Disadvantages sometimes result where a portion of the field of view to be imaged includes a region outside the patient's body, i.e., a region in which radiation passes from the source to the detector, such as the image tube, in vascular imaging systems, without passing through a portion of the patient's body. Often, when the radiation is adjusted to a sufficiently high level to image the patient's body structures, the region or portion of the field of view outside the patient's body will be saturated. For purposes of the present disclosure, any portions of the field of view of the detector in which radiation passes directly to the detector without first passing through the patient's body will be called "air regions".
Where these air regions generate saturation, they will appear in the image as intensely white, or bright. Some persons interpreting the images find this phenomenon to be distracting and to degrade the image. Also, processing information derived from air regions adds nothing to the information obtained about the patient's body, but still occupies a substantial portion of the information processing capability of the system.
Other types of x-ray imaging systems such as digital radiographic systems employing detectors with discrete detector elements, rather than an image tube and television chain, also suffer disadvantages from saturation. The disadvantages suffered by such digital radiography systems include some of those associated with the above described vascular imaging system, and some different disadvantages as well.
In digital radiography, the source directs x-radiation through a patient's body to a detector in the beam path beyond the patient. The detector, by use of appropriate plural discrete sensor elements, responds to incident radiation to produce analog signals representing the sensed radiation image, which signals are converted to digital information and fed to a digital data processing unit. The data processing unit records, and/or processes and enhances the digital data. A display unit responds to the appropriate digital data representing the image to convert the digital information back into analog form and produce a visual display of the patient's internal body structure derived from the acquired image pattern of radiation emergent from the patient's body. The display system can be coupled directly to the digital data processing unit for substantially real time imaging, or can be fed stored digital data from digital storage means such as tapes or disks representing patient images from earlier studies.
Digital radiography includes radiographic techniques in which a thin spread beam of x-rays is used. In this technique, often called "scan (or slit) projection radiography" (SPR) a spread beam of x-rays is directed through a patient's body. The spread beam is scanned. across the patient, or the patient is movably interposed between the spread beam x-ray source and an array of individual cellular detector elements which are aligned along a path. Relative movement is effected between the source-detector arrangement and the patient's body, keeping the detector aligned with the beam, such that a large area of the patient's body is scanned by the spread beam of x-rays. Each of the detector segments produces analog signals indicating characteristics of the received x-rays.
These analog signals are digitized and fed to the data processing unit which operates on the data in a preselected fashion to actuate the display apparatus to produce a display image representing the internal structure and/or condition of the patient's body.
Details of digital radiographic systems are set forth in the following documents, all of which are hereby expressly incorporated by reference:
Lehman, L. A. et al: "Generalized Image Combinations In Dual KVP Digital Radiography", Medical Physics 8:659-667, 1981;
Published European Patent Application No. 83307157.4, published on Aug. 8, 1984 by Gary T. Barnes;
U.S. Pat. No. 4,383,327 issued May 10, 1983 to Robert A. Kruger and entitled "Radiographic Systems Employing Multi-Linear Arrays of Electronic Radiation Detectors ".
One of the advantages of digital radiography and fluoroscopy is that the digital image information generated from the emergent radiation pattern incident on the detector can be processed, more easily than can analog data, in various ways to enhance certain aspects of the image, to make the image more readily intelligible and to display a wider range of anatomical attenuation differences.
One of these image-enhancing processing techniques is automatic gain correction. An example of such gain correction is described in U.S. Pat. Application Ser. No. 798,428, filed on Nov. 15, 1985 by Richard A. Sones, et al. and assigned to the assignee of the present application, and which is expressly incorporated by reference. The use of automatic gain correction, however, gives rise to a particular disadvantage in digital radiography. More specifically, when gain correction is used in digital radiography, distracting streaks sometimes appear in the portion of the image corresponding to the air regions.
Briefly summarized, gain correction involves multiplication of the brightness value produced by a discrete detector element by a gain factor which is uniquely associated with that element. Even where the radiation level sensed across an air region is fairly uniform, processing of image information corresponding to the air region in accordance with the respective gain factors of the individual detector elements will yield differences in image brightness, which will show up in the air region portion of the image as one or more streaks, blotches, or the like. Such would be the case, for example, if two adjacent detector elements both represented a saturation level of radiation, but one had a gain correction factor of 1.1 and the other a gain correction factor of 0.9.
While digital radiographic imaging procedures performed with cellular multi-element detectors do not usually include the taking of "technique" shots such as described in connection with the previously referenced vascular imaging system incorporating the television chain, the digital radiographic system suffers saturation-related disadvantages similar to those of the vascular systems, with respect to the extreme brightness of the air regions, and share the undesirability of requiring the system to process saturation indicating data corresponding to air regions, which contributes nothing to the image of the patient's body.
With respect to the disadvantage of having to process image data corresponding to air regions, the amount of time wasted in processing such air region data varies as a function of the ratio of the area of the imaging system field of view corresponding to the air region, on the one hand, and to that corresponding to the patient data, on the other.
It is an object of this invention to provide system and method for acquiring technique images having particular deliberately added artifacts for indicating whether and where image saturation or near saturation has occurred, and to reduce undesirable results of saturation in air regions.